Injectable sacrificial material systems and methods to contain molten corium in nuclear accidents

ABSTRACT

Systems and methods for injecting a carbonate-based sacrificial material into a nuclear reactor containment for containment of molten corium in severe nuclear reactor accidents are disclosed. Molten corium can be quickly cooled and solidified by the endothermic decomposition of the sacrificial material.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 16/521,887, filed Jul. 25, 2019, entitled “INJECTABLE SACRIFICIALMATERIAL SYSTEMS AND METHODS TO CONTAIN MOLTEN CORIUM IN NUCLEARACCIDENTS,” which claims priority to the benefit of U.S. ProvisionalPatent Application Ser. No. 62/703,132, filed Jul. 25, 2018, entitled“INJECTABLE SACRIFICIAL MATERIAL SYSTEM TO CONTAIN EX-VESSEL MOLTENCORIUM IN NUCLEAR ACCIDENTS,” both of which are hereby incorporated byreference in their entireties.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract No.DE-NA0003525 awarded by the United States Department of Energy/NationalNuclear Security Administration. The Government has certain rights inthe invention.

FIELD

The present disclosure is generally directed to nuclear accidentcontainment, and more particularly to an injectable sacrificial materialsystem and method.

BACKGROUND

The catastrophic nuclear reactor accident at Fukushima created a loss ofconfidence in nuclear energy and a demand for new engineered safetyfeatures that could mitigate or prevent radioactive releases to theenvironment. Molten corium falling into the reactor cavity after thereactor vessel breaches is a major concern in severe accident managementbecause it could result directly breaching of the containment. Corium,also called fuel containing material (FCM) or lava-like fuel containingmaterial (LFCM) due decay heat, is a lava-like material created in thecore of a nuclear reactor during a meltdown accident. It consists of amixture of nuclear fuel, fission products, control rods, structuralmaterials from the affected parts of the reactor, products of theirchemical reaction with air, water and steam, and, in the event that thereactor vessel is breached, molten concrete from the floor of thereactor room. Some new reactor designs employ a core catcher formed ofconcrete and a ceramic sacrificial material to slow the molten flow.

The primary function of the sacrificial material (SM) is to ensureeffective cooling and immobilizing the core melt, eliminate or minimizehydrogen gas release, and maximize radionuclide retention. Existingreactors rely on water to provide cooling. Because existing reactorscannot easily be modified to include these SMs and due to limitationsassociated with these SMs, a solution is needed that can ensureeffective cooling and immobilization of the core melt, eliminate orminimize hydrogen gas release, and maximize radionuclide retention.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to a system for delivering acarbonate-based material within a nuclear reactor containment. Thesystem includes a nuclear reactor contained within the nuclearcontainment, a storage tank containing a mass of the carbonate-basedmaterial, and a fluid delivery system for transporting thecarbonate-based material within the nuclear containment.

The present disclosure is further directed to a method for containingcorium in a nuclear reactor accident within a reactor containment bycontacting a carbonate-based granular material with molten corium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of an arrangement using thecarbonate-based SM during a nuclear accident.

FIG. 2 shows pressures achieved in the reactor containments for variousoperating reactor designs in U.S.

FIG. 3 illustrates two carbonate-based material delivery systemembodiments according to the disclosure.

Wherever possible, the same reference numbers will be used throughoutthe drawings to represent the same parts.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete and will fully conveythe scope of the invention to those skilled in the art.

The present disclosure is directed to systems and methods for injectinga sacrificial material (SM) system to contain and cool molten coriumduring a nuclear accident. In an embodiment, the nuclear accident may bea lower head failure at a commercial light water nuclear reactor (LWR).The SM is a carbonate-based granular material. The sacrificial materials(SMs) ensure effective cooling and immobilization of the core melt,eliminate or minimize hydrogen gas release, and maximize radionuclideretention, thus preventing or minimizing release from containment. TheSM quickly cools the corium mixture while creating an inert gas to formporosity in the solidified corium mixture, such that subsequent waterflooding can penetrate the open pore structure within the corium mixturefor additional cooling. In addition, the SM may form a barrier toprevent the further movement of corium and limit corium-concreteinteractions.

The carbonate-based granular material is an alkaline, an alkali,transition metal carbonate or mixtures thereof. The material may be amineral or a mixture of these minerals. In an embodiment, the alkalinecarbonate material is selected from a group including, but not limitedto calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃), dolomite[CaMg(CO₃)₂] and mixtures thereof. In an embodiment, the alkalicarbonate may be, but is not limited to sodium carbonate (Na₂CO₃),potassium carbonate (K₂CO₃) and mixtures thereof. In an embodiment, thetransition metal carbonate may be, but is not limited to iron carbonate(FeCO₃), manganese carbonate (MnCO₃) and mixtures thereof. In anembodiment, the carbonate-based granular material may be a mineralderived from naturally occurring carbonate rocks or minerals includinglimestone, dolomite, calcite, magnesite and siderite, which aregenerally bulkily available and cheap. In an embodiment, thecarbonate-based, granular material may be prepared from these naturallyoccurring rocks and minerals, for example, by crushing, grinding and/orpalletization. When prepared from mineral deposits, the material mayalso be referred to as mineral-based. The solid carbonate-based granularmaterial has a particle size of sub-millimeters to centimeters indiameter. The particle size (or grain size) can purposely be engineeredto control the rate of carbonate decomposition in the cooling of acorium melt.

FIG. 1 illustrates an embodiment 10 of an arrangement using thecarbonate-based SM during a nuclear accident. As can be seen in FIG. 1 ,a reactor vessel 20 is shown breached and molten corium 24 is shownescaping therefrom. The escaping corium 24 forms a molten pool andcorium crust as shown in the containment vessel 30. Cooling water is incontact with the corium, and slag and/or gas films may be present. Heatis being transported in the molten corium via convection and is alsobeing conducted to the corium crust and slag and gas film.

Carbonate decomposition is highly endothermic (ΔH₀=˜170 KJ/mol forCaCO₃). At the anticipated lower head vessel failure, the injection ofthe carbonate-based SM into the reactor cavity can preferably be carriedout prior to the vessel failure. This geometry allows the molten coriumto fall onto the SM bed to allow the highly endothermic reactions totake place whereby the carbon minerals decompose endothermically,resulting in rapid cooling and thus solidification of the molten corium.In this embodiment, the carbonate-based SM includes CaCO₃ and FeCO₃. Inother embodiments, other carbonate-based SMs according to thisdisclosure may be used.

CO₂ generated from mineral decomposition displaces any preexistingoxygen from the metal oxidation and suppresses the possibility forhydrogen explosion. CO₂ bubbles through the molten corium, creatingporosity and large surface areas that could facilitate subsequentdissipation of radionuclide decay heat by water circulation after coriumsolidification. In addition to CO₂ bubbling, the resulting metal oxidewill mix with the corium, creating a density inversion that promotesfurther mixing of the SM with the corium and accelerating the reaction(see FIG. 1 ).

The carbonate-based SM can include other inorganic compounds which canbe stabilized at high temperature and able to sequestrate highly mobileradionuclides, such as Cs-137, Sr-90, and I-129 or minimize hydrogen gasgeneration. By controlling the composition of the carbonate-based SM,for example, by adding hematite (Fe₂O₃) which can convert metalliczirconium to zirconium oxide by reaction during corium cooling andsolidification, hydrogen gas generation from metallic zirconium can beminimized or even eliminated in subsequent decay heat dissipation bywater circulation. The percentage of Fe₂O₃ to be added is determined bythe mass fraction of metallic zirconium initially contained in coriummelt. Because of its granular form, the carbonate-based SM can beinjected into the reactor cavity and the fluidity of the material andthe rate of CO₂ generated can be controlled as well by the grain size ofthe material.

Calculations were performed to estimate the quantities of the carbonatesrequired to solidify a molten corium at 1000 K for several U.S. reactorplant designs using the concept of endothermic decomposition ofcarbonates by removing heat from molten corium (assuming the moltencorium temperature at 2450 K). For these calculations, the carbonate wasCaCO₃. Its decomposition temperature is at 1098 K. Based on athermogravimetric analysis (TGA), the carbonate starts to decomposebefore the decomposition temperature is reached.

In this calculation, the material property values at 1000 K are assumedconstant. The grain size of carbonate was 5 mm and the carbonate bed hada porosity of 0.37. The granular carbonate was injected into the cavity(or drywell pedestal) at 298 K. No containment leakage was assumed, eventhough there is a nominal leakage rate during operation in a plant.

Table 1 shows the results of these calculations for the six PWR and BWRdesigns types for the U.S. As can be seen in Table 1, the amount ofcarbonate needed for the complete cooling of corium in these reactorcontainments was on the order of 10⁴ to 10⁵ kgs.

Containment Type CaCO₃ Injected CO₂ Generated (Reactor Name) to Cavity(Kg) (kg) Large Dry (Zion) 40,000 2,219.0 Sub-atmospheric (Surry) 60,0001,161.0 Ice Condenser (Sequoyah) 75,000 2,796.0 Mark I (Peach Bottom)280,000 12,440.0 Mark II (LaSalle) 250,000 4,529.0 Mark III (Grand Gulf)350,000 2,749.0

FIG. 2 shows the calculated pressures achieved in the reactorcontainments. As can be seen in FIG. 2 , only the Mark III plant had afinal containment pressure near the ultimate pressure of itscontainment. Note that this calculation does not consider the heatremoval due to containment walls and equipment, and the normal leakageof the containment. The potential gas pressure from the accident isignored.

The present disclosure is also directed to methods for effectivelycooling and immobilizing the core melt, eliminating or minimizinghydrogen gas release, and maximizing radionuclide retention therebypreventing or minimizing release from containment during a nuclearaccident. The method includes injecting a sacrificial material (SM)system to contain and cool corium during a nuclear reactor accident. Inan embodiment, the nuclear reactor accident may be a lower head failureat a commercial light water nuclear reactor (LWR) which includes thepressurized water reactor (PWR) and the boiling water reactor (BWR). TheSM is a carbonate-based granular material.

The carbonate-based material may be injected before, during or aftercorium has breached the reactor vessel. In an embodiment, thecarbonate-based material is injected into the reactor cavity (orpedestal) before the lower head fails. It may be preferred to inject thecarbonate-based material prior to lower head failure for several reasonsincluding, but not limited to:

-   -   The presence of the carbonates may delay the lower head failure        by removing the heat from the lower head during the endothermic        reaction of carbonate decomposition.    -   In case the lower head does fail, the molten corium would fall        onto a carbonate bed and react quickly to solidify the corium        and generate open porosity structures in solidified corium.    -   The decomposition of carbonates also allows the atmosphere of        the containment to be cooled, which may help condense volatile        radionuclide back to the surfaces of solidified corium and the        sacrificial material. Importantly, the generation of CO₂ from        the decomposition would displace the other gases, such as        hydrogen and oxygen, thus reducing the potential for hydrogen        explosions.

The carbonate-based material delivery system may be an active or apassive injection system. Which of these two delivery systems isselected depends on the final size of the carbonate grains used. Thegrain size can range between sub-millimeters and centimeters indiameter. For active systems, the smaller size can be delivered usingpressurized spray systems, such as those commonly used in automotiveindustries for painting. Pressure spray systems use a fluid to injectthe carbonate-based material into the containment. The fluid may be fromliquids, such as water, or pressurized gas, such as CO₂. The larger sizecan be delivered using a particular feed system, such as grain deliverysystems used in agricultural applications. In an embodiment of a passivesystem, gravity can be used to deliver a released carbonate-basedmaterial into the reactor cavity.

FIG. 3 illustrates two carbonate-based material delivery systemembodiments according to the disclosure. As shown in this figure, thesesystems are for the BWR Mark I containment 100. In other embodiments,the systems may be modified for any other containment but still bewithin the scope of the invention. As can be seen in FIG. 3 , a firstactive pressurized gas system 110 is incorporated into the containment100. The pressurized gas system 110 includes a pressurized gas source112, for example, a pressurized gas tank, for forcing carbonate-basedgranular material (not shown) within a material storage tank 114 intothe reactor vessel 150. The storage tank 114 is connected to a deliverynozzle 116 within the reactor vessel 150 via a delivery conduit 115. Thesystem 110 further includes electrical and flow controls (not shown) forcontrolling the release of the pressurized gas, material release, andflow amount, these controls being well understood in the material flowarts. In this exemplary embodiment, the pressurized gas source 112 is apressurized gas tank or cylinder, but in other embodiments, the gassource may be a tank or system located within or partially within thecontainment 100. Additionally, in this exemplary embodiment, thecarbonate-based material is dry, solid granular material. In otherembodiments, the carbonate-based material may be in a slurry, such as byusing water to facilitate material transport.

The pressurized gas system 110 may be modified by the pressurized gascylinder 112 to form a passive system, and in such a manner allow thecarbonate-based material to flow into the reactor cavity (or pedestal)under gravity.

As can be further seen in FIG. 3 , a second active delivery system 120is shown. The system 120 includes a pump 122 for forcing acarbonate-based material (not shown) contained in a material storagetank 124 into the reactor vessel 150. The tank 124 is connected to thenozzle 116 via a fluid conduit (not shown). As in the earlierembodiment, the system 120 further includes electrical and flow controls(not shown) for controlling the release of the pressurized gas, materialrelease, and flow amount, these controls being well understood in thematerial flow arts.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the appended claims. It is intendedthat the scope of the invention be defined by the claims appendedhereto. The entire disclosures of all references, applications, patentsand publications cited above are hereby incorporated by reference.

In addition, many modifications may be made to adapt a particularsituation or material to the teachings of the disclosure withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the disclosure not be limited to the particular embodimentdisclosed as the best mode contemplated for carrying out thisdisclosure, but that the disclosure will include all embodiments fallingwithin the scope of the appended claims.

What is claimed is:
 1. A system for delivering a carbonate-basedmaterial to molten corium within a nuclear reactor containment,comprising: a nuclear reactor contained within the nuclear containment;a storage tank containing a mass of the carbonate-based material; and agravity fluid delivery system for transporting the carbonate-basedmaterial to the molten corium within the nuclear containment vessel;wherein the carbonate-based material is prepared by a method selectedfrom a group consisting essentially of crushing, grinding andpalletization to form a grain size ranging from sub-millimeters to 100centimeters; and wherein the grain size is chosen to control carbonatedecomposition rate and injectivity of the carbonate-based material. 2.The system of claim 1, wherein the carbonate-based material is acarbonate-based material selected from a group consisting essentially ofalkaline, alkali, transition metal carbonates and mixtures thereof. 3.The system of claim 1, wherein the carbonate-based material is analkaline carbonate-based material selected from a group consistingessentially of calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃),dolomite [CaMg(CO₃)₂] and mixtures thereof.
 4. The system of claim 1,wherein the carbonate-based material is an alkali carbonate-basedmaterial selected from a group consisting essentially of sodiumcarbonate (Na₂CO₃), potassium carbonate (K₂CO₃) and mixtures thereof. 5.The system of claim 1, wherein the carbonate-based material is atransition metal carbonate-based material selected from a groupconsisting essentially of iron carbonate (FeCO₃), manganese carbonate(MnCO₃) and mixtures thereof.
 6. A method for containing corium in anuclear reactor accident within a reactor containment, comprising:delivering and contacting a carbonate-based material with molten corium;wherein the carbonate-based material is delivered by a non-pressurized,passive delivery system using gravity to deliver the carbonate-basedmaterial to the molten corium; wherein the carbonate-based material isprepared by a method selected from a group consisting essentially ofcrushing, grinding and palletization to form a grain size ranging fromsub-millimeters to 100 centimeters; and wherein the grain size is chosento control carbonate decomposition rate and injectivity of thecarbonate-based material.
 7. The method of claim 6, wherein thecarbonate-based material is selected from a group consisting essentiallyof alkaline, alkali, transition metal carbonates and the mixturesthereof.
 8. The method of claim 6, wherein the carbonate-based materialis an alkaline carbonate-based material selected from a group consistingessentially of calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃),dolomite [CaMg(CO₃)₂] and mixtures thereof.
 9. The method of claim 6,wherein the carbonate-based material is an alkali carbonate-basedmaterial selected from a group consisting essentially of sodiumcarbonate (Na₂CO₃), potassium carbonate (K₂CO₃) and mixtures thereof.10. The method of claim 6, wherein the carbonate-based material is atransition metal carbonate-based material selected from a groupconsisting essentially of iron carbonate (FeCO₃), manganese carbonate(MnCO₃) and mixtures thereof.
 11. The method of claim 6, wherein thecarbonate-based material comprises hematite (Fe₂O₃) to convert metalliczirconium to zirconium oxide by reaction during corium cooling andsolidification, thus eliminating or minimizing hydrogen gas generationfrom metallic zirconium.
 12. The method of claim 11, wherein thehematite (Fe₂O₃) percentage of the carbonate-based material isdetermined by the mass fraction of metallic zirconium initiallycontained in corium melt.